One method for measuring the concentration of glucose in blood is to cause a reaction between the glucose and the enzyme glucose oxidase. The reaction produces hydrogen peroxide, which can be electrochemically oxidized at positive potentials. The current produced by the oxidation of peroxide is measured and is proportional to the glucose concentration in the blood. A similar scheme may be used to measure other analytes of interest by selecting the appropriate enzyme that will produce hydrogen peroxide in proportion to the concentration of the analyte, e.g., lactate may be similarly measured using the enzyme lactate oxidase.
A long standing problem with these measurement techniques is that of interference from other substances. Blood may contain other components which can oxidize at the same potential as hydrogen peroxide; prevalent among these are urate and acetaminophen. As these or other interfering substances oxidize, the current resulting from their oxidation is added to and indistinguishable from the current resulting from the oxidation of the hydrogen peroxide. An error therefore results in the quantification of the peroxide and consequently the blood analyte.
Many methods have been tried to reduce or eliminate these interferences. Acetaminophen has been particularly difficult to eliminate because, unlike urate, acetaminophen is not charged. One methodology disclosed in the art (see, e.g., Sittampalam, G., and Wilson, G. S., Analytical Chemistry, 55:1608-1610, 1983, and Geise, R. J., et al, Biosensors and Bioelectronics, 6:151-160, 1991) has been to use one or more selectively permeable membranes to prevent the interfering substances from contacting the electrode, although this method is not effective at fully preventing interference.
An alternate method for preventing interference is disclosed in U.S. Pat. Nos. 5,225,064 and 5,334,296. This method decreases the electrical potential of the reaction to a state where urate and acetaminophen are not oxidized. This may be accomplished by employing an enzyme that will reduce hydrogen peroxide, such as a form of peroxidase, in the electrode. In this type of system, the hydrogen peroxide reacts with the peroxidase, in which reaction the hydrogen peroxide is reduced and the peroxidase oxidizes. If the peroxidase is in intimate contact with an electrode's surface, the peroxidase can be reduced at the electrode by imposing an electrical potential. The magnitude of the current required to reduce the peroxidase is proportional to the concentration of the hydrogen peroxide and hence proportional to the concentration of glucose or other analyte of interest. Moreover, the reactions occur at potentials at which urate and acetaminophen do not oxidize and therefore do not interfere.
The art recognizes the use of horseradish peroxidase or fungal peroxidase physically adsorbed to a gold or carbon surface in attempts to prevent interference (see, e.g., U.S. Pat. Nos. 5,225,064 and 5,334,296; Ho, W. O., et al, J. Electroanal. Chem., 351:187-197, 1993; Wollenberger, U., et al, Bioelectrochemistry and Bioenergetics, 26:287-296, 1991; Csoregi, E., et al, Anal. Chem. 66:3604-3610, 1994). However these reports indicate that such use has yielded unsatisfactory results as the magnitude of the electric currents forming the signals in these applications are quite small and the linearity is poor at high peroxide concentrations.
There is great utility, therefore, in the development of an electrode system in which hydrogen peroxide reacts in a measurable way, generating signal currents of usable magnitudes and that are linear to high concentrations, yet which are not subject to interference caused by the oxidation of interfering substances such as urate and acetaminophen.